The reforming of petroleum raw materials is an important process for producing useful products. One important process is the separation and upgrading of hydrocarbons for a motor fuel, such as producing a naphtha feedstream and upgrading the octane value of the naphtha in the production of gasoline. However, hydrocarbon feedstreams from a raw petroleum source include the production of useful chemical precursors for use in the production of plastics, detergents and other products.
Xylene isomers (“xylenes”) and benzene are produced in large volumes from petroleum by the reforming of naphtha. However, neither the xylenes nor benzene are produced in sufficient volume to meet demand. Consequently, other hydrocarbons are necessarily converted to increase the yield of the xylenes and benzene via processes such as transalkylation, disproportionation, isomerization, and dealkylation. For example, toluene commonly is disproportionated to yield benzene and C8 aromatics from which the individual xylene isomers are recovered.
In addition to xylene, some ethylbenzene is produced using the above-noted processes. However, there is a large difference in ethylbenzene (EB) concentration between mixed xylenes produced by transalkylation versus reforming. Modern transalkylation catalysts have a high dealkylation activity, which results in low yield of EB (about 1% of the C8 aromatic fraction). Reforming catalysts, in contrast, have low cracking activity and generate high concentrations of EB (about 14% of C8 aromatics).
In para-xylene purification processes, it is a requirement to convert EB to prevent accumulation in the recycle loops. Xylene isomerization processes may convert EB through either dealkylation or isomerization reactions. The dealkylation reaction converts EB to form benzene, while the isomerization reaction converts EB to xylene. EB isomerization is equilibrium limited reaction and requires increased hydraulic flow, increased energy consumption, and a high concentration of precious metals. The main advantage of EB isomerization is that it provides the highest yield of desirable para-xylene. EB dealkylation is not limited by equilibrium and therefore requires lower capital, lower energy, and much less precious metal. The drawback of EB dealkylation is that the para-xylene yield is much lower than EB isomerization.
In a typical aromatics complex, the mixed xylene produced by both isomerization processes and reforming processes are combined in a xylene splitter. The mixing of these two streams reduces the concentration of EB to a near equilibrium level. Because the feed is at equilibrium there is limited driving force to form xylene from EB using an EB isomerization process. Thus, the production of xylenes remains sub-optimal.
Accordingly, it is desirable to provide improved methods and systems for reforming and transalkylating hydrocarbons. It is further desirable to provide such methods and systems that are able to efficiently convert the produced ethylbenzene to xylenes. Furthermore, other desirable features and characteristics of the presently disclosed embodiments will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.